In MRI systems or nuclear magnetic resonance (NMR) systems, a static magnetic field B.sub.0 is applied to the body under investigation to define an equilibrium axis of magnetic alignment in the region of the body under examination. An RF field is then applied in the region being examined in a direction orthogonal to the static B.sub.0 field direction, to excite magnetic resonance in the region, and resulting RF signals are detected and processed. Generally, the resulting RF signals are detected by RF coil arrangements placed close to the body. See for example, U.S. Pat. No. 4,411,270 to Damadian and U.S. Pat. No. 4,793,356 to Misic et al. Typically such RF coils are either surface type coils or volume type coils, depending on the particular application. Normally separate RF coils are used for excitation and detection, but the same coil or array of coils may be used for both purposes. For multiple surface RF coils for use in NMR, see U.S. Pat. No. 4,825,162 to Roemer, et al., and for multiple volume coils for use in NMR, see U.S. Pat. No. 5,258,717 to Misic, et al.
A quadrature type coil was introduced by Hayes in 1985 to the NMR community. This coil was readily adopted by scientists and engineers in the NMR community for several volume applications (e.g., body, head, knee, wrist). The coil provided a 41% improvement in signal-to-noise ratio (S/N), a reduction of transmit power by a factor of two and a high degree of B.sub.1 homogeneity over the imaging field-of-view (FOV). The principal quadrature mode of this coil has two linear modes, oriented orthogonal to one another. Additional details regarding such a coil design are found in U.S. Pat. No. 4,783,641 to Hayes, et al.
The recent introduction of array coils to NMR has led to commercially available CTL coils for entire spine imaging, and flexible body arrays for torso imaging. These multichannel coils also significantly help reduce scan times. Should there be any brain trauma in conjunction with the c-spine injuries most common in automobile accidents, this necessitates two studies to be performed. Currently, two coils and more particularly two patient settings are required to perform a combined head and neck study (see, e.g., N. Krause, et al. "Quadrature-Head Coil and Helmholtz-Type Neck Coil--an Optimized RF-Antenna-Pair for Imaging Head, Neck and C Spine at 1.0T and 1.5T", SMRM, 7th Annual Meeting, San Francisco, Book of Abstracts, page 845, 1988). A routine MR study takes approximately 45 minutes, including the patient placement. Thus a combined head and neck study approximates an hour-and-half, and requires that the patient be moved between studies for coil replacement. This is uncomfortable especially for claustrophobic patients in general. In addition, prolonged scan times make the contrast-enhanced studies even more difficult to obtain.
In such cases of cranio-spinal trauma, a head and neck coil will help obtain important clinical information without compromising image quality over extended FOV's. This will also be true in the cases of vascular imaging of the carotids originating from the arctic arch and extending to the circle of willis, and for oncological imaging of the head and neck tumors, without moving the patient. A head and neck array will help reduce patient discomfort while reducing the scan times and increase the patient throughput by up to a factor of two in a MR scanner.
Array type coil designs have been disclosed in the prior art. For example, two birdcage resonators (one for the head, another for the neck) have been overlapped for minimum mutual inductance (see FIG. 1). The neck birdcage 10 was asymmetric and had cutouts to accommodate the shoulders of patients. When this asymmetric birdcage was overlapped to the symmetric head birdcage 12, the individual modes were affected differently. Such an array design is discussed in C. Leussler, "Optimized Birdcage Resonators for Simultaneous MI of Head and Neck", SMRM 12th Annual Meeting, New York, Book of Abstracts, page 1349, 1993.
Referring still to FIG. 1, since the anatomy introduced in the two coils was different, the individual linear modes of the two coils 10, 12 were perturbed in different ways, causing the linear modes to misalign and reduce the isolation between modes in a coil. This in turn affected the isolation with the linear mode in the neighboring coil, thus affecting the isolation between coils in the array. Thus, the optimum overlap for one linear mode was not optimum for the second linear mode of the same coil. Moreover, poor isolation between coils affected the coil matching which in turn affected the alignment and isolation of modes in the individual birdcage which in turn affected the isolation between coils in the array. Therefore, optimization of this coil design (tuning, matching, isolation, alignment of modes of coils in the array) was very complex, iterative and often time consuming, thus making the manufacturing process extremely difficult. In addition, this coil design presented added claustrophobia to patients and therefore was not acceptable for clinical imaging.
FIG. 2 illustrates another array design which included a birdcage coil 14 for the head, and a distributed type quadrature planar pair 16 for the neck. (See, e.g., Srinivasan, R, et al. "A Multi-Modal, Split-Top Head and Neck Vascular Array for MRI", SMR 3rd Scientific Meeting, Nice, France, Book of Abstracts, page 977, 1995; and U.S. Pat. No. 5,543,711to Srinivasan, et al.). The neck coils were overlapped with the head birdcage 14 independent of the other. This was possible, because the neck coils were intrinsically isolated from one another. Although this design was more elegant than the above two coil designs, the sensitivity of the coil in the N mode (neck mode) was under that of the commercially available, adjustable whole-volume neck coil. The characterization of this deficit was difficult because of the dilemma of comparing surface type coil S/N to that of the helmholtz type volume coil. In any case, identical clinical scans obtained from the same volunteers clearly showed such a deficit.
The surface coil design emphasized signals from surfaces close to the coil. Therefore, shimming with this coil was difficult and fat suppression was a challenge. The axial neck images displayed un-even signal intensities in the vertical direction. Also, extended FOV sagittal/coronal images of the head and neck displayed a discontinuity at the coil overlap which affected the overall appearance of the image. This also made the windowing of the images difficult. In order to correct for the uneven signal intensities, a new image intensity correction software was employed (based on the RF coil profile), which slowed the image combination process. In addition, the linear modes of the coil were interfaced to the multiple channels of the MR system, which further reduced the image reconstruction process.
In view of the aforementioned shortcomings associated with existing coil designs, there is a strong need in the art for a quadrature array design that provides a high S/N and uniform coverage over the head and neck areas, and which obviates the need for two separate coils and the additional image correction software.